This invention relates to an apparatus for the formation of multi-layer laminates of thermoplastic materials in an extrusion system in which diverse thermoplastic materials are formed into multiple layers of desired thicknesses, and more particularly to an infinitely adjustable extrusion die head for the extrusion of thermoplastics over a wide range of relative thicknesses and viscosities.
This invention is useful in die heads for the manufacture of plastic bottles, pipes, and film as well as plastic coated wires, cables, optic fibers, etc. There are many advantages achieved by the production of multi-layer constructions of thermoplastics as these materials enable a combination of properties not presently obtained in a single material structure. In co-extrusion, two or more diverse thermoplastic materials form separate molten layers which converge and join under pressure within the die to emerge as a single laminated material. Such processes make use of the laminar flow principle which enables the two or more molten layers to join in a common flow channel without intermixing thereof at the contacting interfaces under proper operating conditions.
The present invention therefore is concerned with so-called multiple layer extrusion systems which have come into recent prominent use as they provide a convenient way of melt lamination, co-extrusion coating and the like, but more particularly provide for formation of multiple layers of similar or dissimilar thermoplastics materials. This invention is more particularly concerned with annular die systems. In general, such dies or adapters are designed with an individual flow channel of fixed dimensions for each layer and normally the layers are brought into contact prior to the exit orifice of the die.
Many devices are known to the art for production of co-extruded tubular articles of this type. In the prior art devices, however, there is no provision for infinite adjustment of the cross-sectional areas of one or more of the extrusion passages so as to enable the production of multiple layer thermoplastic laminates wherein the relative thicknesses of the layers can be changed and wherein adjustments can be made to accommodate thermoplastics that differ widely in their relative viscosities.
A co-extrusion die head with fixed gaps defining the cross-sectional areas of the individual resin passages will operate satisfactorily within a relatively narrow range of relative layer thickness and relative viscosities for which it was designed. Attempts to operate such a die head at substantially different relative thicknesses or viscosity ratios will generally lead to visual and/or structural defects in the finished products due to flow instabilities occurring at the interface of the flowing layers. This interfacial instability phenomenon was documented by C. D. Han in Chapter 8 of his monograph "Multiphase Flow in Polymer Processing" (Academic Press, 1981).
The operating range of a co-extrusion die head is chiefly determined by the cross-sectional areas of the individual flow channels at the point of merger of the molten streams. Plastic melts of substantially similar viscosity values will generally merge in a steady and consistent fashion when their individual velocities are substantially similar. For a die head of given cross-sectional areas this guideline defines the relative thickness or proportion of each layer in the finished product. Attempting to make a substantial change in the layer structure merely by changing the flow rate of the individual materials can lead to interfacial instability. If the same die head is to be used with materials of substantially different viscosities, the ratio of stream velocities at the merging point might have a different optimum value in order to avoid interfacial instability. As a result, it may be impossible to maintain the same layer structure produced on the die head with materials of substantially different viscosities from those for which the die head was designed.
A multiple layer die head with fixed gaps is thus limited in terms of the relative proportions and the relative viscosity values of the resin layers which can be produced with the die head.
One prior patent which addresses this limitation is Cloeren U.S. Pat. No. 4,197,069 which discloses a die head for manufacturing multiple layered sheet products. This device utilizes vanes to infinitely adjust the stream velocities at the merging point. This system, however, is limited to a flat die head configuration, and cannot be utilized in die heads with annular flow channels.
Another related device is described in Ando et al. U.S. Pat. No. 4,578,025. This device utilizes replaceable mandrel segments that define certain cross-sectional areas of flow. This system, however, is cumbersome due to the necessity to disassemble substantial portions of the die head in order to change the desired segment of interest, thereby imposing a long optimization and set-up time to achieve a target layer structure.
It is an object of the present invention to provide an improved extrusion die head for the simultaneous extrusion of multiple layers of diverse thermoplastic materials wherein the flow passages at the point or points of intersection in the die head can be adjusted to provide a laminated product having layers of different relative thicknesses and to accommodate thermoplastic materials having a wide range of relative viscosity values.
It is another object of the invention to provide a die head in which the flow passages can be infinitely adjusted.
A further object of the invention is to provide an improved extrusion die head for the simultaneous extrusion of multiple layers of thermoplastic materials of different thicknesses, flow rates and viscosities wherein the adjustments of the passages is relatively easy to accomplish and does not require substantial disassemble of the entire die head.
It is a further object of this invention to provide means to adjust said passage without replacement of any components of the die head.
The present invention is related to the type of die head that includes an annular channel for each resin used in forming the multiple layer plastic article. Each of the channels direct the resin toward the die head outlet orifice. Immediately upstream of the merger point of two or more resin streams, the sidewalls of each annular channel include opposing concentric surfaces which are inclined relative to the longitudinal axis of the die head. The clearance between the opposing inclined surfaces and the channel diameter determine the cross-sectional area of the gap opening. In the die head of the present invention, one of the two inclined surfaces forming the gap can be moved axially so as to increase or decrease the clearance between the opposing surfaces.
The die head provides for axial movement of the inclined surfaces by incorporating the moveable inclined surface into an annular member whose position can be changed axially in the die head. Preferably the annular members are threadably mounted in the die head so that the gap size can be adjusted by rotating the annular members. In one embodiment the annular members are adjustable by drive means which are operable from outside of the die head eliminating the need for disassembly of the die and mandrel.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.